Prescription opioid analgesics are among the frontline treatments for chronic pain. However, inconsistent therapeutic effects and side effects limit their use. Thus, there is a dire need for more effective and safer analgesic drugs to treat chronic pain conditions. One effective strategy for opioid analgesic drug development is to exploit interactions between delta and mu opioid receptors, using fixed-ratio mixtures of delta and mu agonists, with the goal of creating an optimum mixture that produces synergistic analgesic effects but only additive or sub-additive side effects. Drug combinations are common in medicine (e.g., oxycodone + aspirin), and experimental evidence indicates that combinations of delta and mu opioids produce enhanced analgesia with attenuated side effects, such as respiratory depression and sedation. Given that recent clinical and preclinical reports indicate that mu opioids produce bone/joint pathology, this class of compounds may be especially dangerous to prescribe for musculoskeletal chronic pain conditions. The main goal of this renewal is to evaluate delta/mu interactions on antinociceptive and bone/joint pathology outcomes. The primary hypothesis is that delta/mu agonist mixtures will produce synergistic antinociception but only additive or sub- additive bone/joint pathology. Of course, the assessment of these delta/mu opioid combinations requires predictive and reliable preclinical assays of pain. Although pain is typically associated with pain-stimulated behaviors (e.g., withdrawal responses) and pain-depressed behaviors (e.g., decreases in normally adaptive behaviors like feeding, locomotion, exercise), current preclinical measures of pain rely almost exclusively on pain-stimulated behaviors. This approach has several limitations: First, although assays of pain-stimulated behaviors are predictive of acute pain states, they do not parallel the topography of chronic pain behaviors. Second, assessment of chronic pain in clinical medicine relies heavily on quantification of pain-depressed behavior to assess the presence and impact of pain, and effectiveness of treatment. Finally, drugs may decrease pain-stimulated behaviors by producing motor impairment, which results in a false positive treatment effect. During the parent R15 grant cycle, my laboratory developed several assays of acute and chronic pain-depressed behaviors using feeding, locomotion, wheel running and operant responding as endpoints, and we developed an assay of osteoarthritis pain-depressed wheel running in rats. I will extend the findings of the parent grant according to 2 specific aims: (1) Evaluate delta/mu agonist interactions in assays of osteoarthritis pain-depressed wheel running and operant responding, and (2) Utilize in vivo imaging to evaluate delta/mu agonist interactions on bone/joint pathology in rats with/without osteoarthritis, and with/without access to running wheels. Successful completion of these experiments will facilitate discovery of delta/mu mixtures that produce enhanced pain relief with reduced bone pathology. A favorable preclinical therapeutic/side effect profile of delta/mu combinations may indicate that clinical testing in human participants is warranted.